1. Field of the Invention
The present invention relates to a method and apparatus for image formation utilized in a printing unit of a digital copying machine or a facsimile apparatus, or in a digital printer. More particularly, the present invention relates to a method and apparatus of image formation in which an image is formed on a recording medium by emission of developer ink.
2. Description of the Background Art
A method generally referred to as xerography is used in some of the image forming apparatuses in which image signals are output as a visible image on a recording medium such as a sheet of paper. In such an image forming apparatus, an electrostatic latent image is formed by optical writing means on a developing body having electro-optical characteristics, that is, on a photoreceptor. Thereafter, toner as developer particles is attached to the electrostatic latent image, so that the image is developed, or visualized. Finally, the image is transferred to a recording medium such as a sheet of paper, whereby the image signals are provided as a visible image on the recording medium.
In such a conventional image forming apparatus, a developing body having special structure has been necessary for forming the electrostatic latent image. Further, means for writing the electrostatic latent image to the photoreceptor, as well as means for erasing remaining charges on the developing body have been necessary. Further, a complicated structure for transferring the toner image formed on the developing body onto the recording medium is also necessary. Accordingly, the conventional image forming apparatus has a complicated structure, and therefore reduction in size is difficult.
In view of the foregoing, Japanese National Publication No. 01-503221, Japanese Patent Laying-Open No. 07-186436 and so on propose an image forming apparatus employing a different method free of the above described problems. In the image forming apparatus described in these published applications, charged toner is carried on a toner carrier roller, and the toner is directly emitted by Coulomb force to the recording medium, whereby an image is formed. This method of recording images will be referred to as toner emission recording method in the specification.
In the following, a conventional image forming apparatus employing the toner emission recording method will be described. Referring to FIG. 23, the conventional image forming apparatus includes a developer tank 1, a toner carrier 2 carrying toner 4, a toner supply roller 3 and a layer thickness regulating member 5 which are brought in pressure contact with toner carrier 2 and provided inside developer tank 1, an opposing electrode 6 arranged opposing to toner carrier 2 and on a side opposite to a recording medium 7 conveyed in the direction of a fixing roller 8, and a control electrode 9 arranged between opposing electrode 6 and toner carrier 2 and between recording medium 7 and toner carrier 2.
Referring to FIGS. 24 and 25, control electrode 9 includes a plurality of electrodes (X direction electrodes) 10 stacked on each other and parallel to the longitudinal direction of the toner supply roller, a thin film insulator 11 having a thickness of several tens .mu.m, and a plurality of electrodes (Y direction electrodes) 12 in a direction crossing the X direction electrodes 10. At intersections between X direction electrodes 10 and Y direction electrodes 12, toner passage holes 13 are formed.
The image forming apparatus structured as described above operates in the following manner. Here, it is assumed that toner 4 is negatively charged. Toner 4 in developer tank 1 is supplied by toner supply roller 3 to toner carrier 2. At this time, by the friction between toner carrier 2 and toner supply roller 3, toner 4 is negatively charged and supplied to toner carrier 2. Toner 4 which is adhering to toner carrier 2 is conveyed to layer thickness regulating member 5 and again charged by the regulating member 5 and, at the same time, has its thickness regulated to a constant thickness of 10 to 50 .mu.m. Thereafter, toner is conveyed to a position opposing to control electrode 9 by toner carrier 2.
Control electrode 9 is connected to a driving circuit 15 which operates based on a signal from a control circuit 14 generating the signal in accordance with image information. To ones of X direction electrodes 10 and Y direction electrodes 12 which are selected by control circuit 14, Va volt is applied when a dot is to be printed, and Vb is applied when the dot is not to be printed. To toner carrier 2, Vs volt is applied by an external charges 16, and Vt volt is applied to opposing electrode 6 by an external power supply 17. The values Va, Vb, Vs and Vt are predetermined to allow control of toner emission. More specifically, the voltage values are predetermined to allow control of toner emission by electromagnetically varying strength of electric field generated between toner carrier 2 and opposing electrode 6 by the potentials (Va, Vb) applied to each one of the control electrodes.
Toner 4 which is negatively charged and conveyed to the position opposing to control electrode 9 by toner carrier 2 is emitted toward toner passage hole 13, when an electric field stronger than the electric field allowing start of emission is formed as Va volt is applied to X direction electrodes 10 and Y direction electrode 12, as a dot is to be printed. Toner 4 which has been emitted and reached toner passage hole 13 receives the force of the electric field toward recording medium 7 by the opposing electrode 6 to which Vt volt is applied, and the toner is transferred to recording medium 7.
When the dot is not to be printed, Vb volt is applied to either one of or both of Y direction electrode 10 and Y direction electrode 12. Therefore, the strength of the generated electric field does not reach the strength of the electric field which allows start of toner emission, and therefore the negatively charged toner 4 is not emitted to toner passage hole 13.
Referring to FIG. 25, toner passage hole 13 is so arranged as to form four rows of toner passage holes 13 parallel in the longitudinal direction of toner carrier 2, with adjacent dots partially overlapped with each other, so that a line image can be represented by continuous dots. By changing the timing of control of toner passage holes 13 formed parallel to the toner carrier 2, an image is formed. Recording medium 7 on which a visible image is formed is fed to fixing roller 8, and the image is fixed.
In the conventional structure described above, toner emission is said to be controlled by electromagnetically varying the strength of the electric field generated between toner carrier 2 and opposing electrode 6 by the potentials (Va, Vb) applied to the X and Y direction electrodes 10 and 12. Actually, however, the toner may erroneously be emitted when emission voltage Va is applied only to X direction electrode 10 or Y direction electrode 12, and the operation has been unstable.
Further, it has been found that the above described structure suffers from another problem that satisfactory image cannot be stably formed for a long period of time as the toner is deposited on the surface of control electrode 9 or the toner passage hole 13 is clogged by the toner. To ensure stable image formation, the control electrode should be well maintained.
The inventors have found that these problems apt to occur when there is much toner charged in opposite polarity, or much toner weakly charged. Further, it has been found through observation of the state of toner emission that the toner particles travel not particle by particle but in a cluster of a few to several toner particles. When the cluster of toner particles travel, the cluster may be divided during the travel, and particles may possibly move in directions other than the initial intended direction. This may be one of the causes of the above described problems.
In view of the foregoing, a proposal has been made (Japanese Patent Laying-Open No. 8-6383) in which an apparatus for removing toner charged in opposite polarity or weakly charged toner in the developer tank. However, this proposal makes the apparatus complicated, and increases overall cost.
The conventional structure requires a charging apparatus for charging the toner and a fixing roller for fixing the toner adhering to the recording medium. Therefore, the structure is complicated and large. Such an image forming apparatus is mainly used in offices, which generally are positioned in areas where land price or rent is high. Therefore, the image forming apparatus should occupy as small an area as possible.